FIG. 1 illustrates one general solar cell of mass production model using a mono- or polycrystalline silicon substrate. The solar cell includes a p-type silicon substrate 101 obtained by doping monocrystalline silicon with a dopant such as B or Ga. An emitter layer 102 is formed in a light-receiving surface of silicon substrate 101 by heat treatment to diffuse a dopant for imparting n-type conductivity such as P or Sb into silicon substrate 101 in a high concentration. A plurality of extraction electrodes 104 of several hundreds to several tens of microns (μm) wide are disposed contiguous to the emitter layer 102 for extracting photogenerated electric charge from substrate 101. Also, several collector electrodes 105 of several millimeters (mm) wide are disposed for collecting the charges drawn in extraction electrodes 104 and interconnecting solar cells together. While these electrodes may be formed by various methods, one method commonly employed from the standpoint of cost is by printing a metal paste comprising fine particles of metal such as Ag and an organic binder through a screen or the like, and heat treating at a temperature of several hundreds to about 850° C. for bonding to the substrate. On the surface of the substrate opposite to the light-receiving surface, a back electrode 107 of opposite polarity to the light-receiving side electrode is formed by using a metal paste comprising fine particles of metal such as Al or Ag and an organic binder, screen printing, and firing at a temperature of about 700 to 850° C. Between silicon substrate 101 and back electrode 107, an electric field layer 106 containing a dopant for imparting the same p-type conductivity as the substrate such as Al, B or Ga in a high concentration is formed for efficiently extracting charge generated on the back side to the outside. Further, in the region where light is incident on the solar cell, a passivation film 103 is formed for optical confinement. A silicon nitride film which is formed by chemical vapor deposition (CVD) or the like is commonly used as the passivation film.
The passivation film also has a further important function of passivating the silicon surface. In the interior of crystals, silicon atoms are in a stable state due to the covalent bond between adjacent atoms. However, at the surface corresponding to the terminus of atom arrangement, where no adjacent atom to be bonded is available, an unstable energy level known as “dangling bond” appears. Since the dangling bond is electrically active, it captures and extinguishes charge photogenerated within silicon, detracting from the operation of a solar cell. Thus, the solar cells have been subjected to surface passivating treatment or otherwise treated to form an electric field such that photogenerated carriers may not be captured by dangling bonds. A silicon nitride film formed by CVD is widely used because its optical properties are suited for silicon solar cells and because the film itself has a positive fixed charge and also has a high passivation effect due to the inclusion of much hydrogen capable of passivating dangling bonds.
On the other hand, in the emitter layer where electric charge is present at a high density, direct recombination of electrons and holes, known as Auger recombination, becomes outstanding, detracting from the operation of a solar cell. Also, a photon-absorption phenomenon by free charge, known as free carrier absorption, becomes prominent to reduce the amount of photogenerated charge. Accordingly, the dopant concentration of the emitter layer should desirably be kept as low as possible or the depth of the emitter layer should desirably be shallow. However, reducing the dopant concentration, in turn, increases the contact resistance between silicon and metal electrode, giving rise to the problem of an increased resistance loss of generated power. Also formation of a shallow emitter layer is difficult to control, making it difficult to maintain a high production yield on the mass production level. Even if a shallow emitter layer can be formed, the electrode can penetrate through the emitter layer during electrode formation including firing step. This frequently results in a phenomenon that no solar cell performance is available.
Approaches for avoiding these problems include a method of selectively adding a dopant to an electrode-forming region in a high concentration while adding no dopant to a non-electrode-forming region (known as “localized doping”), and a method of adding a dopant in a relatively low concentration (known as “double doping”).
The localized doping structure or double doping structure is generally formed by forming a dielectric film of several hundreds of nanometers (nm) such as a silicon oxide or silicon nitride film on substrate surface as diffusion barrier, opening an electrode-forming portion of the dielectric film by photolithography (see, for example, J. Knobloch, A. Noel, E. Schaffer, U. Schubert, F. J. Kamerewerd, S. Klussmann, W. Wettling, Proc. the 23rd IEEE Photovoltaic Specialists Conference, p. 271, 1993), etching paste (see, for example, JP-A 2003-531807), or laser ablation, and heat treating in a heat-treatment furnace for vapor phase diffusion of a dopant only through the opening. These methods, however, are not suited at all for mass production because they include complex steps or need expensive materials or apparatus.
One simpler method suited for mass production is by premixing a dopant in a conductive paste as the electrode-forming material, as proposed in, for example, D. L. Meier, H. P. Davis, R. A. Garcia, J. A. Jessup, Proc. the 28th IEEE Photovoltaic Specialists Conference, p. 69, 2000. With this method, P is added to Ag paste as dopant, for example, the paste is printed onto a silicon substrate, and the paste is fired at or above the eutectic point of Ag and Si. While the portion of silicon onto which the Ag paste has been applied is once melted and recrystallized upon cooling, P in the Ag paste is taken into the silicon, whereby a high concentration P-doped region is formed immediately below the electrode-forming portion. This method is commonly referred to as “self-doping” and the conductive paste for implementing the method is referred to as “self-doping paste.” The above article reports that the method actually results in a good electrical contact between Ag and Si.
On the other hand, the self-doping paste contains a dopant for forming a self-doped region, an additive for effectively forming a high-concentration doping layer, and the like in relatively high proportions. For this reason, while the self-doping paste provides a good electrical contact between electrode and silicon, the content of metal particles contributing to the conduction of charge extracted from the silicon substrate must be kept low. As a result, the sintered body of self-doping paste has a high interconnect resistance, giving rise to the problem that the output of the solar cell is reduced.